-LRB- CNN -RRB- -- After nearly 40 years of recorded increases , the number of immigrants living in the United States remained flat between 2007 and 2008 , recent statistics released by the U.S. Census Bureau show .

The number of naturalized citizens in the U.S. increased , partly attributed to voter drives for the 2008 election .

According to the Census Bureau 's American Community Survey , the U.S. foreign-born population represented about 12.5 percent of the population in 2008 , down from 12.6 percent in 2007 .

Taking into account the margin of error , it was possible that the immigrant population remained even .

`` Between '07 and '08 there really was n't that much of a change , '' said Elizabeth Grieco , chief of immigration statistics staff at the Census Bureau .

But given the steep upward trend in the foreign-born population since 1970 , no change is big news .

The American Community Survey collects data from about 3 million addresses each year , and provides one of the most complete pictures of the population , according to the bureau .

The survey does n't give a reason for the leveling off , but experts pointed to the economic downturn and the resulting high unemployment as factors behind the shift .

`` The recession has had a significant effect on immigrants ' decisions on whether to come to the U.S. , '' said Michelle Mittelstadt , director of communications at the nonpartisan Migration Policy Institute .

Would-be unauthorized immigrants and legal temporary workers are mostly the ones who have decided to stay put in their home countries for now , Mittelstadt said .

The largest declines in the foreign-born population were in states that were hardest hit by the recession , including California , Florida and Arizona .

Mittelstadt noted , however , that those immigrants already in the United States appear to be staying .

A recent study by the Pew Hispanic Center concluded that emigration from Mexico , the largest source of immigrants to the United States , slowed at least 40 percent between mid-decade and 2008 , based on national population surveys in the United States and Mexico , as well as Border Patrol apprehension figures .

The Mexican-born population in the United States dropped by about 300,000 between 2007 and 2008 , according to census data .

The new Census statistics show that for the first time since the American Community Survey was fully implemented in 2005 , the number of noncitizens decreased , Grieco said .

There were about 21.6 million noncitizens in 2008 , down from 21.9 million in 2007 . The label noncitizens includes both legal residents and illegal immigrants .

Along with the decline in the noncitizen population , however , there was a notable increase in the number of naturalized citizens , Grieco said .

The number of individuals who are naturalized citizens increased to 43 percent of the foreign-born population in 2008 from 42.5 percent in 2007 .

The Census survey matches reports from the Department of Homeland Security on the rise of naturalization applications .

`` Naturalizations grew at a record pace between 2006 and 2008 , with a total of 2.4 million immigrants becoming new citizens in the United States , '' according to a DHS statement .

A significant fee increase imposed in 2007 for naturalization applications and an awareness of citizenship brought on during voter registration drives for the 2008 election help explain the increase , Mittelstadt said .

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Bureau official : `` Between '07 and '08 there really was n't '' much immigration change

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Notable increase in naturalized citizens , Census Bureau 's Elizabeth Grieco says

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Mexican-born population in U.S. dropped by about 300,000 between 2007 and 2008